This invention generally relates to AC-DC transfer standards.
An AC-DC transfer standard provides a DC output voltage approximately equal to the true RMS value of a complex or sinusoidal AC signal (referred to hereinafter as an AC signal). In one type of transfer standard, the AC signal to be measured is applied to a heater element adjacent to or contacting a temperature sensor. One type of temperature sensor is a bimetallic junction thermocouple, which comprises two wires that are joined at both ends. According to the Seebeck effect, heating one junction will induce a current in the wires. The joule heating caused by the AC signal induces a voltage in the thermocouple, which voltage is proportional to the temperature difference between the bimetallic junction adjacent the heater element and the cold junction temperature.
The thermocouple voltage is nulled by adjustment of a potentiometer, which is adjusted to apply an equal and opposite voltage to that generated by the thermocouple. Then, without changing the potentiometer setting, an easily measured DC voltage is substituted for the AC signal. The DC voltage amplitude is increased until the voltage generated by the thermocouple is again equal and opposite to the voltage generated by the potentiometer, as previously adjusted for the AC signal. When this null condition is reached, the amplitude of the DC voltage is measured. This DC amplitude is equal to the RMS value of the AC signal.
As an alternative to a thermocouple, the temperature of the heater wire can also be sensed with a sensing wire having a high temperature coefficient of resistivity. Typically, the sensing wire is wrapped about and electrically insulated from the heater wire. The electrical resistance of the sensing wire is a measure of the temperature of the heater wire. (Further particulars concerning this type of sensing device can be found in application Ser. No. 580,450 entitled "Resistive Sensing Thermal Device for Current Measurement," inventor Fred. L. Katzmann, filed Feb. 15, 1984 and assigned to the same assignee as this application.)
The heater wire is normally made of material having a relatively high electrical resistance, such as nickel-chromium alloy or Evanohm.RTM. (the latter being preferred) about 0.35 to 0.5 mils in diameter. Any current above approximately 7 milliamps through this typical heater wire will anneal the wire and change the DC reversal error, necessitating readjustment of the AC-DC transfer standard. (DC reversal error is discussed in a co-pending patent application entitled "AC-DC Transfer Standard Temperature Sensor Reversal Error Compensation Circuit," U.S. Ser. No. 728,886, filed Apr. 30, 1987, and assigned to the assignee of this application. The contents of that application are incorporated herein by reference.) Additionally, any current greater than approximately 15 milliamps may destroy the heater wire. Thus there is a need for heater wire overload protection.
In one type of known overload protection scheme, the voltage applied to a heater wire is monitored by a bipolar peak-sensing overload protection circuit. Specifically, the gate of a field effect transistor is connected to the heater wire. When the peak voltage level across the heater wire exceeds a certain value, the conduction state of the field effect transistor changes, causing triggering of a silicon controlled rectifier, which causes a relay to disconnect the input voltage from the heater wire. With a typical circuit of this type, approximately one millisecond elapses from the start of the overload transient until the heater wire is disconnected from the applied voltage. The signal voltage must not rise more than 25 percent from its initial value during this disconnect time to avoid damage. Since this is a peak detection system, the proper AC peak voltage amplitude at which the conduction state of the field effect transistor should change may well be above that for a DC signal having an amplitude that can overload the heater element.
In another known overload protection scheme, an infrared sensor monitors the temperature of the heater wire. When an overload occurs, the current through the infrared sensor increases, triggering a silicon controlled rectifier. Due to the thermal inertia of the wire, the infrared overload protection sensor is not a peak detection sensor; additionally, the sensor makes no electrical contact with the heater wire, and therefore is not affected by the frequency or waveshape of the AC input to the heater wire. The reaction time of this infrared sensing circuit is typically on the order of 2 to 5 milliseconds. The drawback of using an infrared sensor is that a relatively large amount of current is needed to trigger the sensor. Thus lower currents, which can overload the heater wire, may not cause triggering of the overload circuit.